1. Field of the Invention
This invention relates to a compact planar type dielectric filter to be mainly used in high frequency radio equipment such as a portable telephone set and the like.
2. Description of the Prior Art
Recently, there is an increasingly growing demand for further down-sizing of a planar type dielectric filter which can be made thinner in structure as compared with the coaxial type being widely used for portable telephone sets.
An explanation follows on the operation of a conventional dielectric filter of a laminated planar type as an example. A conventional planar dielectric filter comprises two thick dielectric layers, a first dielectric sheet on which two coil electrodes are formed, a second dielectric sheet on which one-side electrodes of two parallel plane capacitors are formed, a third dielectric sheet on which the other side electrodes of the two parallel plane capacitors are formed, a fourth dielectric sheet on which a shield electrode is formed, and a dielectric sheet which serves to protect the electrodes, which are laminated from the bottom in the order of the fourth dielectric sheet, one of the two thick dielectric layers, the first dielectric sheet, the other of the two thick dielectric layers, the second dielectric sheet, the third dielectric sheet and the dielectric sheet for electrode protection. In the dielectric filter constructed as above, the parallel plane capacitors are formed respectively between the capacitor electrodes confronting to each other. The parallel plane capacitors are connected through respective side electrodes to the coil electrodes in series to serve to act as a resonance circuit. The two coils are magnetically coupled to each other, and the input/output terminals are taken intermediately of the coil electrodes, thus forming a band-pass filter. (See, for example, Japanese Laid-Open Patent Publication No. 3-72706.)
With the conventional dielectric filter structured as above, if the coil electrodes are disposed close to each other to decrease the distance therebetween for down-sizing, a problem arisen in that a good narrow band band-pass characteristic is not easily realized due to the fact that the magnetic coupling between the resonance circuits becomes too large.